Apart from the aerodynamic design of a wind turbine rotor blade, the quality and weight of the rotor blades are essentially determined by the design of the blade connection to the rotor hub, i.e. the blade root section. What makes the design of the blade connection to the rotor hub a difficult task is the load transfer from the fiber composite structure of the rotor blade into the metal structure of the rotor hub. Such a load transfer is difficult in principle due to the substantially different properties of the materials involved. A known technique for attaching the rotor blades to the hub is to form T-bolt or cross-bolt connections between the blade root and the hub.
However, the rotor loads are concentrated at the blade root portion and the rotor hub. Furthermore, the rotor loads exhibit a highly dynamic load spectrum. Accordingly, specific design margins have to be obeyed both for the bolts and for the root laminate in the blade/hub connection. In particular, the blade bolt design margins may be exceeded so that a larger number of bolts is required for a given load spectrum. However, increasing the number of bolts reduces, in turn, the remaining amount of root laminate between adjacent T-bolt connections. As a result, the blade root connection may be insufficient to withstand extreme loads so that the blade root connection can be damaged or even break off. The above considerations are particularly critical in cases where it is planned to retrofit existing turbines with larger rotor blades.